Oh Wannabeskinny! This sounds like a vacation from h-e-double hockey sticks.
First off, your mother sounds like a GEM. Not everyone is small, skinny, and fashion forward. She sounds like a control freak and a negative word that they wouldn't publish (think Germans in the early 40's). I do not do well with that either, come on, who are we kidding? No one would.
Have you tried having a conversation with her about what you're doing and trying to achieve in your life? Sometime I think that people, especially parents, really do not know what they are doing or how they are acting unless someone tells them. My husband had to 'go off' on my parents once for me since he could visibly see what they were doing to me as I shrunk into a corner one time.
I know this vacation is necessary because come on you want your son to get to know his grandparents and other family. I get that, it's important! but how will you survive? will thinking that your mother is special needs help like it helps me with my parents? can you stand up to her and say, "thank you for your opinion, but this is what I chose to wear now" and move on? Will you tell her about IE and your disordered eating and how you want to change and not be tortured by her or your eating?
This is BIG stuff so maybe this trip it is small steps that you feel comfortable with OR maybe you pack it up and go stay at hotel where you have more freedom over your schedule? Just some thoughts.
Please keep checking in - we're here to help!